1. The Field of the Invention
The present invention relates generally to a buoyancy system for supporting a riser of a deep water, floating oil platform. More particularly, the present invention relates to a buoyancy module with an improved seal.
2. The Background Art
As the cost of oil increases and/or the supply of readily accessible oil reserves are depleted, less productive or more distant oil reserves are targeted, and oil producers are pushed to greater extremes to extract oil from the less productive oil reserves, or to reach the more distant oil reserves. Such distant oil reserves may be located below the oceans, and oil producers have developed offshore drilling platforms in an effort to extend their reach to these oil reserves.
In addition, some oil reserves are located farther offshore, and thousands of feet below the surface of the oceans. Certain floating oil platforms, known as spars, or Deep Draft Caisson Vessels (DDCV) have been developed to reach these oil reserves, examples of which are described in U.S. Pat. Nos. 4,702,321 and 5,558,467. Steel tubes or pipes, known as risers, are suspended from these floating platforms, and extend the thousands of feet to reach the ocean floor, and the oil reserves beyond.
It will be appreciated that these risers, formed of thousands of feet of steel pipe, have a substantial weight which must be supported by buoyant elements at the top of the risers. Steel air cans have been developed which are coupled to the risers and disposed in the water to help buoy the risers, and eliminate the strain on the floating platform, or associated rigging. One disadvantage with the air cans is that they are formed of metal, and thus add considerable weight themselves. Thus, the metal air cans must support the weight of the risers and themselves. In addition, the air cans are often built to pressure vessel specifications, and are thus costly and time consuming to manufacture.
In addition, as risers have become longer by going deeper, their weight has increased substantially. One solution to this problem has been to simply add additional air cans to the riser so that several air cans are attached in series. It will be appreciated that the diameter of the air cans is limited to the width of the well bays within the platform structure, while the length is merely limited by the practicality of handling the air cans. For example, the length of the air cans is limited by the ability or height of the crane that must lift and position the air can. One disadvantage with more and/or larger air cans is that the additional cans or larger size adds more and more weight which also be supported by the air cans, decreasing the air can""s ability to support the risers. Another disadvantage with merely stringing a number air cans is that long strings of air cans may present structural problems themselves. For example, a number of air cans pushing upwards on one another, or on a stem pipe, may cause the cans or stem pipe to buckle.
Furthermore, air cans may develop leaks which reduce their ability to perform, or support the risers. It may be difficult to locate such leaks due to the size of the air cans and their location in the water. As stated above, air cans may be large and difficult to handle. Therefore, it may be difficult to remove and examine the air cans once they are positioned and attached to a riser. In addition, it may be difficult to test the air cans prior to installation.
It has been recognized that it would be advantageous to optimize the systems and processes of accessing distant oil reserves, such as deep water oil reserves. In addition, it has been recognized that it would be advantageous to develop a system for reducing the weight of air cans, and thus the riser system and platforms. In addition, it has been recognized that it would be advantageous to develop a system for increasing the buoyancy of the air cans. In addition, it has been recognized that it would be advantageous to prevent leaks in the buoyancy cans or system. In addition, it has been recognized that it would be advantageous to test a buoyancy system or cans for leaks prior to installation, and to monitor the buoyancy system or cans for leaks during use. In addition, it has been recognized that it would be advantageous to prevent buoyancy systems or cans from developing leaks.
The invention provides a modular buoyancy system including one or more buoyancy modules. The buoyancy modules are vertically oriented, disposed at and below the surface of the water and coupled to a riser or stem pipe to support the riser. The one or more buoyancy modules are sized to have a volume to produce a buoyant force at least as great as the riser.
A first, or upper, buoyancy module may be coupled to a ring structure of an end cap, which in turn may be coupled to the stem pipe or riser. In addition, first and second, or upper and lower, buoyancy modules may be coupled together by a ring structure.
In accordance with one aspect of the present invention, the buoyancy module advantageously may be coupled to the end cap, or the buoyancy modules advantageously may be coupled together, with internal attachments. The buoyancy modules may have interior flanges extending into an interior cavity and abutting the ring structure. Fastening means, such as bolts, may engage the ring structure and interior flange, and couple the interior flange of the buoyancy module to the ring structure to form the interior attachment.
As stated above, an upper end of the upper buoyancy module may be coupled to the ring structure of the end cap, and thus may have an upper flange. In addition, two buoyancy modules may be coupled together with the upper buoyancy module having a lower flange, and the lower buoyancy module having an upper flange, both of which are coupled to the ring structure disposed therebetween.
In accordance with another aspect of the present invention, a test port extends to the seals formed between the flanges and the ring structure. The seal may include a seal member, such as an 0-ring, disposed in a groove between the flange and ring structure. A compressor may be connected to the test port by a fluid conduit to create an increased pressure in the groove. In addition, a pressure gauge may be coupled to the test port to measure any pressure drop in the groove. In addition, a dye may be disposed between the flange and the ring structure to identify any leaks.
In accordance with another aspect of the present invention, the buoyancy modules may be configured to balloon or expand to enhance the seal between the flanges and the ring structures. The buoyancy modules may include a material and may have a thickness that together allow walls of the buoyancy modules to expand outwardly under an internal pressure. The outward expansion causes the walls to pivot slightly about the internal attachment, forcing the flange against the ring structure to enhance sealing between the flange and ring structure.
A method for testing the seal integrity of the buoyancy system includes attaching the flange of the buoyancy module to the ring structure. A pressure differential is created in the groove through the test port. Whether the seal meets predetermined specifications is then determined. Assembling and testing may be performed prior to submerging the buoyancy module. In addition, testing may be performed after submerging the buoyancy module.
In accordance with one aspect of the present invention, the riser may be over 10,000 feet long with an associated weight, and the buoyancy module advantageously may include an elongated vessel with a composite vessel wall. Preferably, the composite vessel wall advantageously has a decrease in weight when submerged between approximately 25 to 75 percent.
Additional features and advantages of the invention will be set forth in the detailed description which follows, taken in conjunction with the accompanying drawing, which together illustrate by way of example, the features of the invention.